1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus having this system.
2. Description of the Related Art
In recent years, with image pickup elements (photoelectric transfer devices) employed for an image pickup apparatus such as a digital camera, high pixilation has advanced. In response to this, high resolution has been demanded for camera lenses (imaging optical systems) employed for an image pickup apparatus which can have a high pixel image pickup element. In order to realize a high resolution camera lens, it is useful for various aberrations relating to image capabilities at a single color (single wavelength) such as a spherical aberration, and a coma aberration to be reduced well, and in addition to this, it is useful for an image obtained when employing white illumination light to have a well-corrected chromatic aberration so as not to exhibit chromatic bleeding.
On the other hand, in order to enlarge a photographing area, there is demand for camera lenses to serve as a zoom lens which can have a high zoom ratio. Generally, further lengthening of the focal length at a zoom position at the telephoto end to obtain a high zoom ratio causes an increase in the chromatic aberration of magnification (lateral chromatic aberration) to occur at a zoom position on the wide-angle side, and also causes an increase in the chromatic aberration of magnification and axial chromatic aberration (longitudinal chromatic aberration) to occur at a zoom position on the telephoto side. Accordingly, in order to obtain imaging capabilities of high image quality, it has been important to appropriately perform not only primary spectrum correction but also secondary spectrum correction for chromatic aberration. Note that herein when referring to corrections or correcting an aberration, a reduction of the aberration or a correction of the aberration is intended.
In addition to this, recently, there is strong demand for reduction in size of the entire camera lens, due to the reduction in size of image pickup apparatuses.
Generally, with a photographic optical system, the more the entire lens length (distance from the first surface to the image surface, also referred to as “entire optical length”) is reduced, the more chromatic aberrations such as axial chromatic aberration and chromatic aberration of magnification occur, and also optical capabilities deteriorate. In particular, with a telephoto-type optical system, the longer the focal length is, the more chromatic aberration expands, and also the more chromatic aberration due to reduction of the entire lens length increases.
As for a method for reducing such occurrences of chromatic aberrations, an achromatic method using an extraordinary partial dispersion material, or an achromatic method using a diffraction grating have been widely known.
With a telephoto-type optical system, a positive lens made up of a low dispersion material having extraordinary partial dispersion such as fluorite, and a negative lens made up of a high dispersion material are commonly employed for reducing chromatic aberrations as a forward lens unit in which the positions where a paraxial marginal ray and a paraxial chief ray pass through are relatively high as to the optical axis. Various types of such a telephoto-type optical system have been discussed (see Japanese Patent Publication No. 1985-49883 (corresponding to U.S. Pat. No. 4,241,983), Japanese Patent Publication No. 1985-55805 (corresponding to U.S. Pat. No. 4,348,084), and Japanese Patent Laid-Open No. 1999-119092 (corresponding to U.S. Pat. No. 6,115,188)).
Note that a paraxial marginal ray is a paraxial ray incident in parallel with the optical axis of an optical system with the height from the optical axis as 1 when normalizing the focal length of the entire optical system to 1. Also, a paraxial chief ray is a paraxial ray passing through the intersection between the incident pupil and optical axis of an optical system, of rays incident by −45 degrees as to the optical axis when normalizing the focal length of the entire optical system to 1. With the incident angle of the optical system, the clockwise rotation measured from the optical axis is assumed to be positive, and the counterclockwise rotation is assumed to be negative. Note that an object is assumed to be present on the left side of the optical system, and the ray incident to the optical system from the object side is assumed to proceed from the left to the right.
Also, a telephoto-type optical system has been known where a diffractive optical element is employed for correcting a chromatic aberration without employing an extraordinary partial dispersion material (see Japanese Patent Laid-Open No. 1994-324262 (corresponding to U.S. Pat. No. 5,790,321), Japanese Patent Laid-Open No. 1994-331887 (corresponding to U.S. Pat. No. 5,629,799), Japanese Patent Laid-Open No. 1997-211329 (corresponding to U.S. Pat. No. 5,872,658)).
Japanese Patent Laid-Open No. 1994-324262 (corresponding to U.S. Pat. No. 5,790,321) and Japanese Patent Laid-Open No. 1994-331887 (corresponding to U.S. Pat. No. 5,629,799) have discussed a telephoto-type optical system having an F number of F 2.8 or so of which a chromatic aberration is corrected relatively appropriately by combining a diffraction-type optical element and a refractive-type optical element.
In addition to these, as for a material, which can have a correction function of a chromatic aberration related to a diffractive optical element, a liquid material exhibiting properties of equivalently high dispersion and also equivalently extraordinary partial dispersion has been known, and an achromatic optical system employing this material has been known (see U.S. Pat. No. 4,913,535 and Japanese Patent Laid-Open No. 2002-156582 (corresponding to U.S. Pat. No. 6,496,310)).
Note that as for a telephoto-type zoom lens, a zoom lens, which can have a 4-unit configuration made up of lens units of positive, negative, positive, and positive refracting power in order from the object side to the image side in which a chromatic aberration is corrected with glass having extraordinary dispersion properties, has been known (see Japanese Patent Laid-Open No. 2001-194590 (corresponding to U.S. Pat. No. 6,404,561) and Japanese Patent Laid-Open No. 2002-62478 (corresponding to U.S. Pat. No. 6,594,087)).
Also, a zoom lens made up of lens units of positive, negative, positive, negative, and positive refracting power in order from the object side to the image side has been known (see Japanese Patent Laid-Open No. 1998-90601 (corresponding to U.S. Pat. No. 6,025,962)).
Glass having great extraordinary dispersion properties such as fluorite or Product Name S-FPL51 manufactured by Ohara Inc. is low in a material refractive index. Accordingly, it is useful for performing the desired secondary spectrum correction to equivalently enlarge the curvature of a lens surface to enforce the refracting power of the lens.
Consequently, a lens made up of such a material has a tendency of enlarging the lens thickness thereof. Also, with an optical system for aiming at suitable chromatic correction effects as apochromat, in order to realize the desired chromatic correction effects, it can be necessary in some circumstances to increase the number of lenses and cemented lens surfaces, and consequently, the entire optical length has been apt to increase to secure the lens thickness.
With the telephoto-type optical systems employing fluorite or other related or equivalent materials as known by one of ordinary skill in the relevant art discussed in Japanese Patent Publication No. 1985-49883 (corresponding to U.S. Pat. No. 4,241,983), Japanese Patent Publication No. 1985-55805 (corresponding to U.S. Pat. No. 4,348,084), and Japanese Patent Laid-Open No. 1999-119092 (corresponding to U.S. Pat. No. 6,115,188), a chromatic aberration is correctable in the case of the entire optical length being set relatively long. However, it can be difficult to correct occurrence of a chromatic aberration accompanied with reduction of the entire optical length. This is because this technique simply reduces chromatic aberrations generated by the forward lens unit itself using low dispersion and extraordinary partial dispersion included in the material such as fluorite. In order to correct chromatic aberrations increased along with reduction of the entire optical length, for example, with an optical system employing low-dispersion glass, which can have a great Abbe number such as fluorite, the chromatic aberration thereof is not changed unless the refracting power of the lens surface is greatly changed. This makes it difficult to satisfy correction of a chromatic aberration and correction of various aberrations such as a spherical aberration, coma aberration, and astigmatism, contemporaneously.
On the other hand, diffractive optical elements have a sufficient correction function of a chromatic aberration. However, the diffraction light of unnecessary diffraction order other than the diffraction light of design diffraction order actually employed becomes a colored flare light, which deteriorates image formation capabilities.
Also, the achromatic optical system employing a liquid material exhibiting relatively extraordinary partial dispersion properties has no function for agglutinating an optical lens, and also needs to have a configuration for sealing. Also, this system also has a problem where properties such as a refractive index and dispersion properties change along with change in temperature, and accordingly, we can say that environmental capabilities are far from being sufficiently satisfied.